One of the current trends in the automobile industry is to lower vehicle weight to help achieve better fuel economy, thus helping to meet fuel economy standards and to offset the higher fuel prices. Another trend is that there is a broader range of vehicle models, which in turn reduces the volume of vehicles produced on a per model basis. Sport utility and crossover vehicles remain popular and typically include fairly heavy rear liftgates making this part of the vehicle a target area for weight reduction. Liftgates are traditionally made from stamped steel panels that are heavy and have a high tooling cost. Traditional steel liftgates are expensive investments, heavy, take up a lot of OEM floor space in areas from stamping plant, body shop, paint shop, and trim line. Further, steel liftgates have limited styling flexibility, take a lot of time to tool, and have corrosion concerns. Sheet Molding Compound (SMC) is an alternative to steel for the inner and outer panels of the liftgate. Using SMC has several manufacturing concerns related to the material and process. Steel and SMC liftgates have a mass penalty over thermoplastics. There are also styling restrictions with traditional sheet metal components. Thermoplastic composite type materials used for liftgate applications also have difficulty meeting customer performance specifications.
Another concern with the manufacture of liftgates is that typical liftgates are manufactured as a relatively flat or smoothly contoured panel, with structural reinforcements such as ribs added onto the panel. This will also add weight and increase manufacturing complexity as well and when thermoplastics are used there are read through areas where the ribs are placed which must be dealt with by design modifications or expensive processes such as gas assist injection molding. Ribs are also weaker and do not carry the load through the liftgate panel. Recently magnesium inner reinforcement panels have been used with an outer polymer skin in order to reduce weight. While such panels are an improvement in weight, this is an expensive solution. Another concern with typical liftgates is that the structural reinforcements are steel or larger steel structures adding weight and increased manufacturing complexity. Another concern is typically reinforcement material is used for reinforcement in the structural areas and attachment structures are fixed using bolts. However, the use of bolts does not provide a continuous attachment structure and improved strength since there is distance between the bolts. Yet another concern with the manufacture of liftgates is that typical liftgates are manufactured as relatively solid with no access features such as access doors added into the panel to allow for easy access for general maintenance and repair of built in components.
A known 2008 Nissan Murano composite liftgate system helped to satisfy the weight savings and the tooling cost concerns, but utilized a typical bolt in small steel reinforcement at the latch which secures one end of the liftgate to the vehicle. This does not meet the higher load requirements desired in some applications, such as the latch pull test. A known Nissan Rogue composite liftgate system utilizes a steel one-piece outer panel and steel brackets. This does not improve density, painting efficiency, hold tighter tolerances, is more expensive and complex to manufacture, and adds weight to the liftgate/vehicle.
Accordingly, there exists a need for a composite liftgate which is both lightweight, as well as structurally sound enough to meet various load requirements, while being more mass and cost effective.